The latin letter is used in mathematics to define complex numbers, where represents the square root of negative one. The symbol is also used in computing and linear algebra as an index variable.
A complex number is an extension of the real number line where in addition to the "real" part of the number there is a complex part of the number. The properties of complex numbers are useful in applied physics as they elegantly describe rotation.
A variable is a core concept in algebra where a symbol, usualy a lower case latin letter, is used as a placeholder in a math expression.
The capital Latin letter I is used in math to represent the identity matrix.